Can a male get bacterial infection (e.x. e. coli) via oral sex if partner has no STI? If so, how common is that? Can risk be reduced without condom?

E coli is not a risk. E coli is every where, including in your drinking water and is not a concern. If your partner does not have any STI, you should be okay. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.